This invention relates generally to the treatment of the surface of carbon materials for the purpose of improving the bond (shear strength) between the carbon and a proposed matrix material. The invention is applicable to carbon surfaces of all shapes and sizes. It is particularly important for filaments, multifilament tows, thin strips, and the like (hereinafter referred to as carbon fibers). The following discussion is directed to treating filaments as a typical example.
One of the prevailing problems in the field of carbon-reinforced composites is the bond or shear strength between a carbon reinforcement and a matrix material. The problem exists whether the matrix is a plastic resin or a metal matrix. Historically, it has been very difficult to create a tenacious bond because carbon surfaces are not easily wetted by the common matrix materials in use or proposed. Further, in some cases, the carbon surface reacts with the matrix material.
A large number of surface treatments or wetting agents have been proposed. A process for modifying the surface characteristics of carbonaceous fibrous material using molecular oxygen is described in U.S. Pat. No. 3,754,947. U.S. Pat. No. 3,989,802 describes a process for treating carbon fibers with sodium dichromate and sulfuric acid. Boron and boron compounds are widely-used treatments for carbon fibers. One form of such treatment is described in U.S. Pat. No. 3,672,936. Oxygen gas, in combination with radio frequency, is proposed for a treatment for carbon fibers in U.S. Pat. No. 3,634,220. The list is extensive. Specifically, in connection with metal matrices, carbon fibers have been treated with titanium carbide, silicon carbide, and boron carbide.
It is an object of the present invention to provide a specific form of a silicon carbide treatment for a carbon surface.
It is another object of the invention to provide a carbon-rich silicon carbide treatment for a carbon surface.
It is yet another object of the invention to form a carbon-rich layer on a carbon surface wherein the percentage of silicon to carbon varies from zero at the carbon surface and increases towards one at the exterior surface of the carbon-rich layer.
It is still another object of the invention to disclose a vapor deposition process for depositing a carbon-rich layer on a carbon surface for the purpose of improving the sheer strength of the carbon material relative to a matrix material.
It is hypothesized that silicon carbide is particularly sensitive to the presence of non-stoichiometric silicon carbide or impurities. I. T. Kendall, Journal of Chemical Physics, Vol. 21, pg. 821 (1953). Since both Kendall and K. Arnt & E. Hausmanne in Zeits Anorg Chem., Vol. 215, pg. 66 (1933) have found no evidence of non-stoichiometric silicon carbide, it is hypothesized that the excess carbon appears in the silicon carbide as an impurity. The properties of silicon carbide are particularly sensitive to the presence of impurities such as carbon.
Though the precise structure of carbon-rich silicon carbide may not be known with certainty, regions where quantitatively there is an excess of carbon have been observed. It is also theorized that some of the silicon also occurs in a free state.
The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in conjunction with the accompanying drawings, in which: